I have an R31 with an RB25 Neo.
In no way do I claim to know everything, so - to my knowledge...
The turbo has a VG30 style exhaust housing and the compressor wheel is made of a nylon composite material, making it more fragile. Most people say that 10psi is the safe limit.
The R34 internals are probably a little stronger overall then the R33. They are known to have GT-R RB26 style conrods (mine did when it was pulled apart) which are good for the power level you want. Also has GT-R style shim & bucket solid lifters instead of hydraulics.
The stock ECU has an overboost protection which I think comes in at 12 or 14psi(?), I think it basically richens the hell out of the mixture and retards the timing all the way.
Aftermarket ECU I'm not sure, because I don't know if the R34 motors have the same issue with R33 RB25s where they break piston ringlands. I guess if you're not worried about rebuilding it then wind the boost up and tell us how much psi you had in it when something breaks
215rwkw? Don't really know for sure. They're smaller than R33 injectors (R34 are 340cc vs R33 370cc or something like that)
Nismo 440cc?
Apexi PowerFC is very popular, also the new Vipec range.
RB20 gearboxes are very similar to series 3 R31 RB30 boxes. With the power you want you'd be better off with an R33 or R34 gearbox. If you did that you'd have to get a custom tailshaft made. Mine is a single piece using a Z32 300ZX twin turbo gearbox yoke and bigger universal joints.
Australian R31 diffs are pretty weak. I drove on mine for about a year with the Neo and the gears inside the diff centre pretty much collapsed. Had that rebuilt to stock Silhouette LSD specs with a Pintara 4.11 crownwheel & pinion and now 2 years later it's groaning and whining like it's the original diff. I don't drift the car, do burnouts or drive like an idiot, it's just weak. The fix is a Kaaz LSD centre. $1500 last time I heard. Have fun trying to break it after that
I suppose if you drive like an idiot then fairly quickly you'll have to replace the diff, then either the gearbox or the universal joints.
Have no experience with KKRs. You get what you pay for I guess?